Human embryonic stem (ES) cells (Thomson, J. A. et al. Science 1998, 282, 1145-1147) and induced pluripotent stem (iPS) cells (Takahashi, K. et al. Cell 2007, 131, 861-872), which proliferate infinitely and differentiate into numerous cell types, have been serving as valuable tools for basic biological research and as promising resources for regeneration therapy. Despite advances, substantial challenges remain for clinical application of stem cells. One safety concern has been posed by the appearance of a teratoma in animal models transplanted with cell samples containing a small amount of undifferentiated stem cells. Strategies to detect and ablate undifferentiated stem cells are required for safer stem cell therapy.
For the detection of human pluripotent stem cells, antibodies against SSEA-4 (stage-specific embryonic antigen 4) have extensively been used (Shevinsky, L. H. et al. Cell 1982, 30, 697-705). SSEA-4 is a glycolipid that is expressed on the cell surface of early embryos and presented selectively on the surface of human ES and EC (embryonic carcinoma) cells by unknown reasons (Henderson, J. K. et al. Stem Cells 2002, 20, 329-337). Other markers of human stem cells include Oct3/4 and Nanog, transcription factors required for the maintenance of undifferentiated states of stem cells and downregulated upon differentiation (Rosner, M. H. et al. Nature 1990, 345, 686-692, and Mitsui, K. et al. Cell 2003, 113, 631-642). Although their antibodies are highly useful in detecting pluripotent cells, these unstable protein tools suffer from their high cost and careful procedures including fixation and permealization of cells. Another routine marker of human stem cells is alkaline phosphatase (Thomson, J. A. et al. Proc. Natl. Acad. Sci. USA 1995, 92, 7844-7848). Although the assay for its enzymatic activity represents a simple and highly useful method for detecting stem cells, a major concern is its specificity to pluripotent stem cells because this house-keeping enzyme is expressed widely in a number of other cell types.